If you look at the ingredients for Alka-Seltzer, you will find that it contains citric acid and sodium bicarbonate (baking soda). When you drop the tablet in water, the acid and the baking soda react -- this produces the fizz.
Shake and Bake A new and increasingly widespread technique also known as one pot, this condenses the anhydrous method's many steps and requires only a plastic soda bottle, the size of which depends on the quantity of meth the cook intends to make.
Never inject shake n bake...EVER! Hell you really shouldn't even be using shake n bake to be honest, but if you have to, never inject it! Shake and bake, dramatically more so than other recipes of meth, will have much more of the chemicals used to make it left in the final product.